Devices and methods for minimizing start up delay in transmission of streaming media

ABSTRACT

Devices and methods are provided for minimizing the startup delay of streaming media transmitted through networks. A server maintains a portion of the media stream stored in an initial burst transmit buffer. At startup, the stored portion is transmitted at a rate higher than the fixed frame rate, exploiting the full available bandwidth. The initial burst transmission fills up the de-jitter receive buffer at the receiving end faster, thereby shortening the startup delay. Then transmission is switched to the regular rate, from the regular buffer. A variable bit rate transcoder is optionally used for the data of the initial transmission. The transcoder diminishes the size of these frames, so they can be transmitted faster. This shortens the start up delay even more. A receiver has a buffer with a fill level started at a value lower than a final value. This triggers the beginning of play out faster, further shortening the delay time. The fill level of the de-jitter receive buffer is then gradually increased to a desired final value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to the field of data transmission overnetworks, and more specifically to devices and methods for minimizingthe startup delay of streaming media.

2. Description of the Related Art

Networks are increasingly used to transmit data. The data can be forvarious media, such as audio or video, and is usually organized ingroups that are called frames. When such data is transmittedcontinuously, it is also called streaming media. The transmission caneither be for live, or for canned media. Either way, the transmissionhas to be in real-time, or in delayed real-time.

A fundamental requirement for streaming media is that it has to beplayed at a constant rate. Packet networks, however, may transmit dataasynchronously. Therefore, for streaming media to be transmitted overpacket networks, strict timing requirements have to be met in order toachieve high-quality media play out. Furthermore, where there are tworelated but separate streams of media (such as audio and video), theyhave to be time-synchronized.

The bandwidth of the channel is intended to be wider than what isrequired by the streaming media. The channel bandwidth, however, isoccasionally subject to congestion. This causes play out of streamingmedia to exhibit the problem of network jitter, if played out inreal-time. This means that what is played out might be momentarilydiscontinued, if the bandwidth of the network is momentarily constrictedto less than the required bandwidth of the streaming media.

Referring to FIG. 1, the problem of network jitter has been solved inthe art by introducing a de-jitter buffer at the receiving end. A serveror proxy 110 transmits data to a client or receiver 160 through anetwork 150. The server 110 includes a path 112 for data received from asource, and transmitted to the network 150. Typically, the server 110includes a buffer in the path 112, such as a transmit buffer 114.

Importantly, the client 160 includes a fixed size de-jitter receivebuffer 162. The buffer 162 first fills up to its size, and then startsplaying out. While playing out, the buffer 162 is emptied at the samerate as it is filled. Constrictions in bandwidth affect only thefullness of the buffer, not the play out, therefore preventing networkjitter. The larger the size of the buffer, the more the network jitterphenomenon is abated.

A problem generated by the de-jitter receive buffer 162 is a phenomenoncalled startup delay. The phenomenon is now described in more detailbelow.

Referring to FIG. 2, the fullness of the buffer 162 is plotted against atime axis as a line 210. Initially the buffer 162 fills up to its sizefy, which it reaches at time point 212. For illustration purposes, theline 210 is shown in an approximate manner as a ramp. However, the lineis actually piecewise linear, shaped more like a stair.

After reaching its full size fy, playout begins. Then the buffer 162ideally remains at the same fullness level. Again, that is shown as astraight line only for illustration purposes. The fullness level buffer162 may temporarily go lower than the ideal.

It will be appreciated that the larger the size fy is, the longer timeit takes to reach that level. The time between zero and time point 212is called the startup delay. The startup delay causes the play out to bein delayed real-time, compared to when the data is received.

The startup delay can be as long as one second or more, which is aproblem. It is even more of a problem when the user is channel surfing,i.e. switching quickly from one channel to another. With every newchannel switch, there is another startup delay.

BRIEF SUMMARY OF THE INVENTION

The present invention overcomes the problems and limitations of theprior art.

Generally, the present invention provides devices and methods thatdiminish the startup delay of streaming media.

In one embodiment, the invention maintains a portion of the media streamstored in an initial burst transmit buffer of the server. At startup,the stored portion is transmitted at a rate higher than the fixed framerate. This exploits the additional, unused bandwidth of the network.This initial transmission fills up the de-jitter receive buffer at thereceiving end more quickly, thereby shortening the startup delay. Thentransmission is switched to the regular rate.

In addition, a variable bit rate transcoder is optionally used for thedata of the initial transmission. The transcoder diminishes somewhat thesize of these frames, so they can be transmitted faster. This way thestart up delay is shortened even more.

Moreover, the fill level of the de-jitter receive buffer may be startedat a value lower than a final value. This speeds up the beginning ofplay out, further shortening the delay time. The fill level of thede-jitter receive buffer is then gradually increased to a desired finalvalue.

The invention will become more readily apparent from the followingDetailed Description, which proceeds with reference to the drawings, inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a server transmitting data to a client overa network.

FIG. 2 is a time diagram that demonstrates why the phenomenon of astartup delay occurs in the arrangement of FIG. 1.

FIG. 3 is a block diagram of a server made according to the inventiontransmitting data over a network to a client made according to theinvention.

FIG. 4 is a flowchart illustrating a first method according to theinvention.

FIG. 5 is a time diagram comparing the improved startup delaysaccomplished by various embodiments of the invention to the startupdelay of FIG. 2.

FIG. 6 is a flowchart illustrating a second method according to theinvention.

FIG. 7 is a time diagram illustrating the changing fill level of ade-jitter buffer according to the invention.

FIG. 8 is a diagram illustrating how the improved start up delaysafforded by the present invention depend on a ratio of a demandedbandwidth over the available network transmission bandwidth.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

As has been mentioned, the present invention provides devices andmethods for minimizing the startup delay of streaming media. Variousaspects and embodiments of the invention are now described in moredetail. The system of the invention can be a server, a client, or acombination of them.

Referring to FIG. 3, a device according to the invention is a server310, which is also known as a proxy 310. As shown, the server 310 can beeither a full server, or a portion of a server. The server 310 is fortransmitting data over a network 150, such as a packet network, to aclient 360. The client 360, which is also known as a receiver 360,includes at least one de-jitter buffer.

The server 310 includes a regular path 312 for transmitting to thenetwork 150 data received from a source. Preferably, the transmit buffer314 is located in the regular path 312, for buffering data from thesource. Transmission through the regular path 312 is intended to be atthe regular rate, such as at a fixed frame rate r required by thestreaming media in question. The frame rate r may be, for example, 100frames per second for all audio, or 25 frames per second for video. Thestreaming media may have a constant or a variable transmission rate.

The server 310 also includes a burst path 322. The burst path 322 is fortransmitting data received from the source at a burst rate. The burstrate is higher than the regular rate.

The server 310 most preferably includes an initial burst transmit buffer324 in the burst path 322. The initial burst transmit buffer 324 is forbuffering the data received from the source, above and beyond theoperations of transmit buffer 314. The initial burst transmit buffer 324is also for transmitting the buffer to data through the network 150 tothe client 360 at the burst rate.

The server 310 also includes a switch 330. The switch 330 is forselecting to transmit data to the network 150 from either the regularpath 312, or the burst path 322. After the switch, the regular path 312and the burst path 322 are merged.

The server 310 preferably also comprises a control unit 335. The controlunit 335 is primarily for controlling the switch 330. The control unit335 can be implemented as a processor, software in a processor, aseparate circuit, etc.

The server 310 preferably also includes a monitor that measures anamount of the data that is output through the burst path 322. Thatmeasure is most conveniently a number of frames, the same number as arestored in initial burst transmit buffer 324. This monitor can beimplemented in connection with the initial burst transmit buffer 324,such as by software. Alternately, this monitor can be implemented inconnection with the control unit 335. The control unit 335 switches theswitch 330, when a preset measure of the data is output through theburst path 322.

The server 310 moreover includes a bandwidth monitor that can measurethe bandwidth of the network 150. This feature can be optionallyimplemented in conjunction with control unit 335. This feature is usedto provide an estimate of how fast data is being received at the client360.

Additionally, the server 310 can include a fill level controller thatcontrols the fill level of the initial burst transmit buffer 324. Thefill level can be controlled according to the monitored bandwidth.

The server 310 preferably also comprises a transcoder 340. Thetranscoder 340 is made as is known in the art, and is for transcodingthe buffered streaming media that this output through the burst path322. The transcoder 340 could well be placed exclusively in the burstpath 322. It is preferred, however, to place the transcoder 340 wherethe regular path 312 and the burst path 322 are merged. This way, thetranscoder 340 can be additionally used for transcoding the subsequentregular transmission, if it is so desired. Optionally and preferably,the transcoder 340 transcodes the data of the initial burst transmitbuffer 324, if the monitored bandwidth of the network 150 becomes lessthan a preset bandwidth.

A client 360 according to the invention receives streaming media fromthe network 150. In other words, the client 360 receives the data thatencodes the streaming media. The client 360 can be any device thatreceives streaming media from a network, such as a network appliance.

The client 360 has a receive de-jitter buffer 364 for receiving thestreaming media. In addition, it includes a variable bit rate decoder366. It is preferred that the receive de-jitter buffer 364 has achanging fill level. It is preferred that the fill level is increasedgradually, as is described later. It should be noted, however, that theserver 310 of invention does not need the client 360 to have a variablesize de-jitter buffer.

Referring to FIG. 4, a method of the invention is described withreference to flowchart 400. The method is for a server to retransmitstreaming media to a network. The streaming media is encoded in data,and ultimately destined for a client.

According to an optional box 410, the available bandwidth oftransmission is monitored. This can be accomplished by monitoring theavailable bandwidth of the network.

According to an optional box 420, a size is adjusted of a first portionof streaming media. This size can be the number of frames that are to bestored in an initial transmit buffer, such as buffer 324.

According to a box 430, a first portion of the streaming media isreceived along a burst path of the server. The size of the first portionmay have been adjusted as per the above. Preferably, the first portionis continuously updated, such as for live media.

According to an optional box 440, the streaming media received along theburst path is stored in an initial burst transmit buffer, such as buffer324.

According to an optional box 450, it is inquired whether the monitoredbandwidth is less than a preset bandwidth. If it is, then according tobox 455, the first portion of the streaming media is transcoded. It canbe transcoded as it is being played out.

According to a box 460, the first portion of the streaming media(whether transcoded or not) is transmitted to the network through theburst path at the burst rate. The burst rate is as high as permitted bythe bandwidth of the connection.

According to a box 470, a second portion of the streaming media isreceived from the source. The second portion is received along a regularpath of the server, which is distinct from the burst path at least inpart.

According to a box 480, the server is switched to output data from theregular path, instead of the burst path.

According to a box 490, the second portion is transmitted to the networkthrough the regular path. It is transmitted at a regular rate, which islower than the burst rate. The regular rate is intended to be the normalframe rate of the streaming media.

Advantages of the invention can be now appreciated with reference toFIG. 5. The fullness of the buffer 364 is plotted in a vertical axis504, against a time axis 510. FIG. 5 is not necessarily to scale.

The performance of the prior art systems, which was explained in detailwith reference to FIG. 2, is presented again in FIG. 5. That was line210, which gave a start-up time at time point 212.

The initial burst transmit buffer 324 transmits faster than the regularrate of line 210. The resulting performance is given by line 326, whichhas a higher rate of ascent, because it is using the full bandwidth ofthe network 150. Line 326 levels out after the buffer 364 reaches itsfull level fy. This happens at time point 328, at which play out istriggered. This occurs earlier than time point 212, which corresponds toa shortening of the start-up delay. Again, line 326 is straight onlyideally.

Moreover, transcoding the first portion further accelerates how fast thebuffer 364 reaches its nominal fullness level fy. That is because thefullness is measured in terms of frames, and the transcoded firstportion includes less data per frame. This additional feature isdepicted by line 342, which has an even higher rate of ascent than line326. Accordingly, line 342 reaches the fullness level fy at time point344, which is even earlier than time point 328, thus further shorteningthe start-up delay. When this feature is implemented, it is true thatthe first few frames are of a lower quality, because of the transcoding.This does not matter that much, however, because that is also the timethat the end user is getting used to the new stream of information.

Referring now to FIG. 6, a second method of invention is described withreference to flowchart 600. The second method is for client to receivestreaming media over a network.

According to a box 610, the client receives data that encode thestreaming media from the network.

According to a box 620, the received data is stored in a de-jitterbuffer. This increases a fullness of the buffer.

According to a box 630, it is inquired whether the fullness has reacheda first fill level. If it has, then according to a box 640, play out ofthe stored data is initiated.

According to a box 650, the fill level of the de-jitter buffer ischanged while playing out the stored data. Preferably, it is increasedgradually up to a final fill level. In other words, play out isinitiated before the fullness has reached the final fill level.

Referring to FIG. 7, the fill level of buffer 364 is plotted againsttime by line 710. Up until time point 712, the fill level equals fx. Thefill level, or buffer size, of buffer 364 is gradually increased,preferably linearly. It reaches a final fill level fy at time point 716.

The advantage of changing gradually the fill level of the de-jitterbuffer will be appreciated by referring back to FIG. 5. Assuming thatboth the initial burst transmit buffer 324 and transcoder 340 areenabled, the fullness of buffer 364 will be given by line 714. Thisstarts identically from the origin 0, until he teaches a level fx attime point 712. This initiates play out, at the time even earlier thantime point 344. This corresponds to the yet fastest start-up delay ofall the contrasted schemes.

After time point 712, as the fill level is increased, so is the fullnesslevel. As seen before, the fill level will reach level fy at time point716. The fullness would also reach fy, at a time point 718. In general,time point 718 occurs later than time point 716.

It should be noted that both time points 716 and 718 occur after timepoint 212, but that does not matter. The play out has been initiated attime point 712, which shortens the start-up delay.

It should be further noted that the fill level fx started increasing attime point 712, i.e. when the initial fill level fx was reached. That isnot necessary, however. The fill level of buffer 364 can startincreasing to its final level before or after time point 712.

The invention offers its highest advantage when a high bandwidth isavailable for the initial, burst transmission. This is now quantifiedfor the various embodiments discussed above.

Referring now to FIG. 8, the start-up delay is plotted against a ratioof BS/BI. BS is the transmission rate required by the streaming media.BI is the actually available bandwidth. For values larger than 1, nosustained streaming media transmission is meaningful.

The start-up delay of the prior art is given by line 810. Line 810corresponds to the performance of the prior art system, discussed abovewith reference to FIG. 2. In other words, the start-up delay is thesame, regardless of how much available bandwidth BI there is over therequired bandwidth BS.

Once the ratio of BS/BI is known, a ratio point 806 is chosen on theratio axis. Then a vertical line 510 is drawn from ratio point 806. Itshould be noted that this line 510 is also the horizontal line 510 inthe diagram of FIG. 5. Accordingly, where line 810 intersects line 510,time point 212 is defined.

If only the initial burst transmit buffer 324 is implemented, thebehavior of the system is characterized by line 830. Line 830 intersectsline 510 at time point 328.

If also the transcoder 340 transcodes the data stored in buffer 324,then the behavior of the system is characterized by line 840. Line 840intersects line 510 at time point 344.

If, in addition to the above, the buffer fill level is increasedgradually from fx to fy, then the behavior of the system ischaracterized by line 850. Line 850 intersects line 510 at time point712.

As can be seen from the vertical axis of FIG. 8, the start-up delaybecomes smaller, as more and more features of the invention areimplemented. In addition, as more bandwidth is available, the line 510shifts closer to the left. Then the time points 712, 344, and 328 slidedown accordingly, corresponding to a diminishing startup delay. In otherwords, the more bandwidth becomes available, the better the inventionworks in minimizing the start-up delay.

A person skilled in the art will be able to practice the presentinvention in view of the description present in this document, which isto be taken as a whole. Numerous details have been set forth in order toprovide a more thorough understanding of the invention. In otherinstances, well-known features have not been described in detail inorder not to obscure unnecessarily the invention.

While the invention has been disclosed in its preferred form, thespecific embodiments thereof as disclosed and illustrated herein are notto be considered in a limiting sense. Indeed, it should be readilyapparent to those skilled in the art in view of the present descriptionthat the invention can be modified in numerous ways. The inventorregards the subject matter of the invention to include all combinationsand subcombinations of the various elements, features, functions and/orproperties disclosed herein.

The following claims define certain combinations and subcombinations,which are regarded as patentable. Additional claims for othercombinations and subcombinations of features, functions, elements and/orproperties may be presented in this or a related document.

1. A server for transmitting data over a network to a client having ade-jitter buffer, the server comprising: a regular path for transmittingdata received from a source at a regular rate; a first buffer in theregular path for buffering data from the source prior to transmission tothe client; a burst path for transmitting data received from the sourceat a burst rate higher than the regular rate before playout at theclient distinct from the regular path at least in part; a second bufferin the burst path for buffering data from the source prior totransmission to the client, and for transmitting the buffered data tothe client at the burst rate before playout at the client; and a switchfor selecting to transmit data from one of the regular path and theburst path.
 2. The server of claim 1, further comprising: a control unitfor switching the switch.
 3. The server of claim 2, further comprising:a monitor that measures an amount of the data is output through theburst path, and wherein the control unit switches the switch when apreset measure of the data is output through the burst path.
 4. Theserver of claim 1, further comprising: a network bandwidth monitor; anda controller that controls a fill level of the second buffer accordingto the monitored bandwidth.
 5. The server of claim 1, furthercomprising: a transcoder for transcoding the buffered streaming mediaoutput through the burst path.
 6. The server of claim 1, furthercomprising: a network bandwidth monitor; and a transcoder fortranscoding the buffered streaming media output through the burst pathif the monitored bandwidth becomes less than a preset bandwidth.
 7. Theserver of claim 1, the regular path for transmitting data received fromthe source over the network at the regular rate.
 8. A server forretransmitting streaming media comprising: means for receiving a firstportion of the streaming media from a source along a first path; meansfor buffering the first portion prior to transmission to a client andoutputting the buffered first portion to the client on the networkthrough the first path at a first rate before playout at the client;means for receiving a second portion of the streaming media from thesource prior to transmission to the client along a second path distinctfrom the first path at least in part; and means for transmitting thesecond portion to the client on the network through the second path at asecond rate lower than the first rate.
 9. The server of claim 8, furthercomprising: means for switching to outputting from the second path, fromoutputting from the first path.
 10. The server of claim 8, furthercomprising: means for storing the first portion.
 11. The server of claim8, further comprising: means for monitoring a bandwidth of the network;and means for controlling a size of the first portion according to themonitored bandwidth.
 12. The server of claim 8, further comprising:means for transcoding the first portion.
 13. The server of claim 8,further comprising: means for monitoring a bandwidth of the network; andmeans for transcoding the first portion if the monitored bandwidthbecomes less than a preset bandwidth.
 14. A method for a server toretransmit streaming media comprising: receiving a first portion of thestreaming media from a source along a first path of the server;buffering the first portion prior to transmission to a client andoutputting the buffered first portion to the client on the networkthrough the first path at a first rate before playout at the client;receiving a second portion of the streaming media from the source priorto transmission to the client along a second path of the server distinctfrom the first path at least in part; and transmitting the secondportion to the client on the network through the second path at a secondrate lower than the first rate.
 15. The method of claim 14, furthercomprising: switching the server to outputting from the second path,from outputting from the first path.
 16. The method of claim 14, furthercomprising: storing the first portion in an initial burst transmitbuffer.
 17. The method of claim 14, further comprising: monitoring abandwidth of the network; and controlling a size of the first portionaccording to the monitored bandwidth.
 18. The method of claim 14,further comprising: transcoding the first portion.
 19. The method ofclaim 14, further comprising: monitoring a bandwidth of the network; andtranscoding the first portion if the monitored bandwidth becomes lessthan a preset bandwidth.